1. Field of the Invention
The present invention relates to a wavelength converter, and more specifically to a converter for converting the optical wavelength in transmitting a signal.
2. Description of the Related Art
Optical communications systems have become popular as networks for transmitting a large volume of information. In the optical communications systems, a signal is normally transmitted using a light having a specific wavelength. Recently, a transmission system based on a WDM (wavelength division multiplex) technology has attracted considerable attention. The WDM technology is realized by transmitting a plurality of signals through a single optical transmission line using different optical wavelengths.
A wavelength converter is used in, for example, a transmission system based on the above described WDM technology, and converts the wavelength of a light which transmits a signal. That is, the wavelength converter outputs a signal transmitted through a light having the wavelength .lambda.1 along the optical light having the wavelength .lambda.2. FIGS. 1 through 3 show examples of the existing wavelength converters.
FIG. 1A shows a wavelength converter based on a gain saturation effect of an input light in a semiconductor optical amplifier SOA (semiconductor optical amplifier). The modulation method based on the gain saturation effect is normally called a cross gain modulation (XGM). With this configuration, a light (input signal light) having the wavelength .lambda.1 intensity-modulated according to a transmission signal, and continuous waves (CW) having the wavelength .lambda.2 are input to a semiconductor optical amplifier 501 to obtain as an output from the semiconductor optical amplifier 501 a light having the wavelength .lambda.2 intensity-modulated according to the above described transmission signal. Thus, the wavelength of an input signal light is converted into the wavelength of an output light from a light source 502. That is, if a wavelength of the output light from the light source 502 is variable, then the wavelength of an input signal light can be converted into a desired wavelength.
FIG. 1B shows a wavelength converter based on the cross-gain modulation of a semiconductor laser. With this configuration, a semiconductor laser 511 for generating a light having the wavelength .lambda.2 based on the specific oscillation frequency is used. When a light having the wavelength .lambda.1 intensity-modulated according to a transmission signal is input to the semiconductor laser 511, the distribution of the wavelengths of the gain of the semiconductor laser 511 fluctuates. As a result, the oscillation light of the semiconductor laser 511 is intensity-modulated. That is, a light generated by the semiconductor laser 511 is intensity-modulated by an input signal light.
FIGS. 2A and 2B show a wavelength converter based on the cross phase modulation (XPM). With this configuration, two devices (for example, semiconductor optical amplifiers) whose refractive index and transmittance fluctuate depending on the input light intensity and the supplied electric current are used. The continuous wave having the wavelength .lambda.2 is input to the two devices, and the light (input signal light) having the wavelength .lambda.1 intensity-modulated according to a transmission signal is input to one of the devices. As a result, these two devices output the lights having the wavelength .lambda.2 with different phases. The phase difference depends on the intensity of an input signal light. Therefore, when the output from these two devices is coupled using an optical coupler, etc., the light having the wavelength .lambda.2 is intensity-modulated according to the input signal light having wavelength .lambda.1.
FIG. 3A shows a wavelength converter using a semiconductor laser and an interferometer. With this configuration, a semiconductor laser 521 for generating a light having the wavelength .lambda.2 is used. Then, a light having the wavelength .lambda.1 intensity-modulated according to a transmission signal is input to the semiconductor laser 521 to make the distribution of the wavelength of the gain of the semiconductor laser 521 fluctuate. As a result, the frequency of the output light from the semiconductor laser 521 can be modulated. After the output light passes through an optical band pass filter 522, the light is converted into an intensity modulated signal using an interferometer 523, for example, a Mach Zehnder interferometer, etc.
FIGS. 3B and 3C show a wavelength converter designed to convert an optical signal temporarily into an electric signal. With the configuration shown in FIG. 3B, a light having the wavelength .lambda.1 intensity-modulated according to a transmission signal is converted into an electric signal using an optical receiving element 531. The electric signal is a transmission signal, and is used to drive an emission element 532 for generating a light having the wavelength .lambda.2. Thus, the light having the wavelength .lambda.2 is intensity-modulated according to the transmission signal. With the configuration shown in FIG. 3C, continuous waves having wavelength .lambda.2 are generated using a light source 541, and the continuous waves are input to an optical modulator 542. Then, the light having the wavelength .lambda.2 is intensity-modulated by applying the electric signal obtained by the optical receiving element 531 to the optical modulator 542.
Reference Document:
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However, it is difficult to improve the extinction ratio using the wavelength converter shown in FIGS. 1A and 1B. For example, to improve the extinction ratio using the wavelength converter shown in FIG. 1A, an optical amplifier and an intermediate light source are required and an adjustment of them is troublesome. Furthermore, since it requires an additional check process, there arises a problem of a bad yield and a high cost.
Since the wavelength converter shown in FIGS. 2A and 2B has an optical coupler to couple lights although it is able to improve the extinction ratio, there is a considerable optical loss in resultant wavelength (.lambda.2 in the above described example) after conversion.
With the wavelength converter shown in FIG. 3A, it is very difficult to adjust an interferometer and the resultant wavelength is fixed. Furthermore, with the wavelength converter shown in FIGS. 3B and 3C, it is hard to realize a smaller and less expensive system.